Beyond that the kernel developers value both correctness and performance. Most of these bugs involve multiprocessing and a huge amount of effort has gone into getting rid of them global locks, but even they aren't easy to reason about at that scale - I think you're missing the common thread in "deadlock" and "Global Interpreter Lock". Pretending that a world could exist where we can simply reason about something like the Linux kernel and then using it to state that devs can simplify or improve the logic is taking a completely unsupported cheap shot and the disclaimer about the real world doesn't change that. I'm not even sure what your motivation for the equivalent of "if you pretend that friction & drag don't exist it's really easy to figure out aerodynamics, so plane designers need to do better" (hint, it's not) is, but it certainly isn't to inform, contribute, or facilitate the debate.
If you don't make rules as to how the parts are to behave, how are writers of device drivers going to know how to make a working driver? Tweak it until it doesn't hang the system in a few days?
And how are you going to fix that deadlock you find during debugging? You may know (simple exampe) that thread T1 holds lock L1 while trying to get L2, while thread T2 holds L2 while trying to get L1, but without guidance on how things are supposed to work, how do you even know whether to fix T1 or T2?
IMO, the more complex the system, the greater the need for simpe rules that limit what different parts can do.
Chances are there will still be lockup bugs, but that is not a reason not to try and avoid them.
I probably live in a dream world, but I would say software bugs of the 'causes deadlock in a OS that is used in millions of devices' type should (maybe even must) be avoided by reasoning about the code. If you find that you cannot reason about the whole system at the time, you either simplify it (python's GIL is a nice extreme example; you never read about race conditions inside Python's core) or you improve your logic.
In my world, you should only need the debugger for hardware related bugs, and then, it often is a combination of intuition and sheer perseverence that solves the issue. Especially if timing is involved, looking at the system can make the problem go away. But even there, having thought about the system will help a lot. For example, your reasoning might have said "between two X-es, we always get at least one Y, so…". You can turn that into a hypothesis that can be tested easily (in theory. Practice can be quite different)
On the other hand, if you must use modern PC hardware, you get a lot of unavoidable complexity (multiple interrupt levels, drivers that you cannot all check do compliance with your model of how drivers work, etc), so you may end up with the case where reasoning about the code isn't really possible.